1. Field of the Invention
1. Field of the Invention
The present invention relates to positioning systems for moving a member between positions in a minimum time in response to a position input command.
2. Description of the Prior Art
A typical positioning application to which the present invention relates is the positioning of a data recording head or heads over a selected track of a magnetic disk file. The aspect of this positioning operation which is of interest is the movement of heads between tracks, known as the "track access" or "seek" operation, as opposed to the "track follow" operation which maintains the heads in position over a selected track.
Time optimal motion between tracks implies maximum acceleration and deceleration of the heads within the physical constraints of the system. To control such motion and bring the heads accurately to rest on the target track, feedback control of head velocity is normally employed. Velocity control offers a higher performance than position control because velocity is the time derivative of position. A typical system for controlling a disk file head access operation is described in an article entitled "Design of a Disk File Head-Positioning Servo" by R. K. Oswald (IBM Journal of Research and Development, Nov. 1974, pp. 506-512).
In the system described in that article and in similar systems employed in many commercially available disk files, an access operation is controlled by means of a generated reference velocity trajectory representing the required velocity of the heads for deceleration at the maximum attainable rate to a state of rest over the target track. A velocity transducer (or tachometer) measures the actual velocity of the heads and the measured velocity is compared with the reference velocity trajectory and amplified in an error amplifier to provide a velocity error signal. The velocity error signal is applied to control the head actuator, typically a voice coil motor, to cause the actual velocity to follow the deceleration curve as closely as possible. Initially, actual velocity is low and the heads are accelerated under open loop (saturated) conditions until the actual velocity equals the reference velocity. When actual velocity exceeds the reference velocity, the sign of the velocity error changes and reverse current is applied to the actuator. The reverse current is controlled as a function of the velocity error to cause the head velocity to follow the reference velocity trajectory accurately. Considerable accuracy is possible since, generally, a high bandwidth velocity measurement is available with very little lag. Typically, a velocity signal is derived from incremental position signals provided by an external position transducer linked to the head motion or, by a servo head and dedicated servo surface on one of the disks.
However, the velocity feedback loop is Type 1 and the reference velocity trajectory is approximately a ramp so that the velocity error can never be completely eliminated. The magnitude of the velocity error is dependent on the overall gain of the feedback loop which also affects the bandwidth. A compromise is necessary between the reduction of velocity error and the limitations on feedback loop gain and bandwidth imposed by the mechanical resonances of the system. If the gain is more than unity at a resonance frequency, the system will be unstable. This problem is becoming increasingly acute with the need to improve access times and with the higher frequencies inherent in increased track density.
In the process control art, control systems are known which employ both feedback and feedforward control. U.S. Pat. Nos. 3,657,524 (Bakke) and 3,758,762 (Littman) describe such systems. In the system described in the Bakke patent, for example, a process is coarsely controlled by means of a manipulated variable to produce a desired change in a measurable controlled variable of the process. A control and feasible response means is responsive to a command signal, defining a set point of the process, to provide a control signal, in accordance with a predetermined model of the process, for controlling the manipulated variable in such a way as to bring the process to the set point. This is "feedforward" control. Simultaneously, the control and feasible response means provides a feasible response signal, representing the predicted response of the controlled variable to the change of the manipulated variable in accordance with the "feedforward" control signal. The feasible response signal is compared with the actual measured response of the controlled variable to provide an error signal. This error signal is summed with the fed forward manipulated variable to provide minor feedback control of the process. The problems of the process control art are somewhat different from those of the position control systems with which the present invention is concerned. In process control there is often a long delay between alteration of the manipulated variable and the process response, which makes feedback control alone impractical. Further, the command signal may change before the previous one has been executed. Thus, although "feedforward" plus feedback control is described for process control applications, there is no suggestion in the cited patents of an application to position control or discussion of the specific features of feedforward plus "feedback" position control to which the present invention relates and which will become apparent below.
To complete the discussion of the prior art, U.S. Pat. No. 3,958,109 to Doherty for a "Universal Modularized Digital Controller" describes a position control system particularly suitable for gun control where command signals are presented in rapid succession. This patent shows a digital system for computing essentially a velocity reference function in response to applied commands and fed back position signals. The computation of the velocity reference involves a component referred to as "additive compensation ("feedforward" velocity)" which is essentially a cumulative function of input commands. This component is used to modify the primary velocity reference signal. It is not employed as a direct input to the power drive for effecting the desired movement, nor does it vary with time in the absence of further commands. The digital velocity reference signal including the additive compensation is converted to an analog function and compared with a fed back velocity measurement to produce a velocity error signal. The velocity error signal alone is applied to control the power drive.
Thus, Doherty shows only feedback control and does not show true "feedforward" control where a predicted function of a manipulated variable (the power drive input) is applied directly to control the power drive so as to produce a desired change in position.